1. Technical Field
The present invention relates to nuclear magnetic resonance apparatuses and, more specifically, to low magnetic field nuclear magnetic resonance apparatuses.
2. Description of Related Art
Nuclear magnetic resonance (NMR) is a phenomenon involved with precession of the magnetic spin of an atomic nucleus arising from resonance of the magnetic spin of the nucleus under a magnetic field when the magnetic field is applied to the atomic nucleus. Magnetic resonance imaging (MRI) is a non-invasive technique of imaging the inner part of an imaging sample. The MRI is widely used as a medical diagnostics tool to image the inner part of human body.
The main magnet in a conventional NMR/MRI is required to have a highly uniform magnetic field of as small as 0.1 and as large as several Tesla (T). Therefore, the main magnet generating the magnetic field is large in volume and high in cost.
Low magnetic field NMR/MRI divides the magnetic field generated from the main magnet of a conventional NMR/MRI into two components: a prepolarization magnetic field and a readout magnetic field. This way, the readout magnetic field may operate while reducing its intensity from tens of microTesla (uT) to several uT.
Low magnetic field NMR/MRI includes a prepolarization coil for generating the prepolarization magnetic field and a readout coil for generating the readout magnetic field. The prepolarization coil-generated magnetic field magnetizes, or prepolarize, the imaging sample. The readout magnetic field is then applied to the imaging sample after complete removal of the prepolarization magnetic field, allowing a readout device to measure the magnetic resonance signal before the magnetization of the imaging target is relaxed off. Thus, the prepolarization coil has only to generate a strong prepolarization magnetic field with significantly relaxed uniformity requirement compared to that of the main magnet of the conventional NMR/MRI systems. Moreover, while the readout coil need to provide a uniform magnetic field, it needs only to generate a weak magnetic field, it may be simple in structure and low in cost. As the intensity of the readout magnetic field is reduced, the readout signal frequency corresponding to Larmor frequency proportional to the intensity of the magnetic field is reduced from several kilohertz (kHz) to hundreds of hertz (Hz).
Therefore, it is possible to measure phenomena not previously measurable with conventional high magnetic field NMR/MRI. For example, low magnetic field NMR/MRI is able to dramatically reduce distortion in the image resulting from the presence of metal inside of or adjacent to the imaging sample. Hence the low magnetic field NMR/MRI may be reasonably applied to a person with metal prostheses. Likewise, low magnetic field NMR/MRI may be able to non-invasively obtain images from inside metal cans. In this way, low magnetic field NMR/MRI may operate as an apparatus for determination or identification of physical properties of an imaging sample in addition to conventional security screening devices using X-ray. With relaxed requirement for the uniformity of the prepolarization magnetic field, low magnetic field NMR/MRI is not limited to the shape of hollow cylinder around the imaging sample and is able to adopt an open design, for example, it may adopt more flexible shapes, like thick cake shape, to place an imaging sample below.
A conventional power MOSFET device is the usual choice for controlling the electric current from a power supply to provide intermittent current to the prepolarization coil. When the power MOSFET device turns the current off, magnetic induction energy from the current already flowing through the prepolarization coil is dissipated at the power MOSFET device as heat through avalanche breakdown-induced resistive heating. This simple method may be able to constitute the driving circuit of the prepolarization coil. However, this method suffers from high energy loss since the magnetic induction energy in the prepolarization coil is dissipated as heat for every prepolarization period. Also, when the power MOSFET device turns on and the current is applied to the prepolarization coil, rising time of the current may depend on the relaxation time constant of an L-R circuit formed by the prepolarization coil and the driving circuit. Due to the characteristics of the prepolarization coil which must generate a strong magnetic field using the current provided by the driving circuit, the prepolarization coil needs a high inductance value (Lp), in the order of hundreds of millihenry (mH). Accordingly, the relaxation time constant of the L-R circuit can become as long as a few seconds. As a result, a pulse length of current providing a prepolarization magnetic field increases accordingly.
In the method of controlling the prepolarization coil using a power MOSFET device, magnetic induction energy generated at the prepolarization coil in each pulse is entirely dissipated as heat by the power MOSFET device. The magnetic induction energy may be as low as a few kilojoule (kJ) (e.g., 2 kJ with 100 A current on a coil of Lp=200 mH) and can easily be tens of kJ (e.g., 18 kJ with 300 A current on the same coil), which requires a large cooling apparatus to remove the heat from the dissipated energy. For this reason, the heating serves as a significant factor to determine the limit of current which may flow to the prepolarization coil.
In low magnetic field NMR/MRI, there is no serious degradation in the image quality from the relaxed uniformity requirement of the prepolarization coil compared to the main magnet of a conventional MRI with comparable main magnet field strength. Therefore, the prepolarization coil may be geometrically simpler than the main magnet coil. Further, since the prepolarization coil is usually driven at a relatively low current, it is typically made of resistive copper wire. The prepolarization coil may generate a magnetic field of about 0.1 to 0.5 T depending on the shape, size, or the position of the object to be magnetized. Generally, current flowing into the prepolarization coil is tens to hundreds of amperes (A). Low magnetic field/very low magnetic field NMR/MRI requires intermittent application of prepolarization magnetic field of usually about 0.1 T or more.
In the case where the prepolarization coil is made of resistive copper conductor, there are two approaches used to decrease the dissipative heating caused by the resistance of the prepolarization coil. One is to increase the sectional area of the conductor. In the case where the prepolarization coil operates at room temperature, resistive heating caused by the conductor resistance may reach a few to tens of kilowatt (kW). Thus, a heavy-duty coil cooling device is necessarily required. Increasing the sectional area of the prepolarization coil can reduce resistive heating of the prepolarization coil when the prepolarization coil is operating at room temperature. In this case, the volume of the prepolarization coil increases accordingly and a cooling device may still be required to maintain the prepolarization coil at room temperature.
The other approach is to cool the prepolarization coil to decrease specific resistance of the copper conductor. If the coil is cooled to below −100 degree centigrade so that copper resistivity decreases to a fraction of its room-temperature resistivity, the coil cooling effect increases significantly since the coil dissipates that much less heat due to its reduced resistivity. To achieve this, the prepolarization coil is generally cooled by submerging it in liquid nitrogen. A dewar for the prepolarization coil may be needed to keep the prepolarization coil submerged in liquid nitrogen during operation. The dewar for the prepolarization coil cools heat generated from the prepolarization coil via vaporization of the liquid nitrogen. In addition, the dewar for a prepolarization coil can maintain a temperature of the coil at vaporization point of the liquid nitrogen, i.e., 77 Kelvin (K) unless the prepolarization coil is exposed above the surface of the liquid nitrogen. Since copper resistivity at 77 K (i.e., vaporization point of liquid nitrogen) is much lower than at room temperature, the prepolarization coil can be considerably smaller. On the other hand, the amount of liquid nitrogen consumed in cooling the dissipated heat from the prepolarization coil is tens of liters (L) per kilowatt-hour (kWh). (22.5 L/kWh to be specific) Thus, the size of the dewar must be large enough to contain the amount of liquid nitrogen necessary to maintain the prepolarization coil submerged during operation. The above-mentioned problems must be addressed in the case where the prepolarization coil is made of a resistive conductor.